1. Field of the Invention
The present invention relates generally to an electromagnetic machine for converting mechanical energy into electrical energy and vice versa. In particular, the present invention relates to a reciprocating linear electromagnetic machine comprising a stator, a coil mounted in the stator for generating magnetic flux, and a plunger having a configuration of variable magnetic properties for the center and outer rows of magnets arranged for reciprocation with respect to the stator.
2. Description of the Prior Art
Linear electromagnetic machines are known which comprise a stator having a plurality of axially spaced coils for generating a magnetic flux and a permanent magnet plunger reciprocating with respect to the stator. Examples of such machines are found in U.S. Pat. Nos. 4,349,757 to Bhate and 4,454,426 to Benson. A machine employing a single coil mounted in the stator is disclosed in U.S. Pat. No. 4,827,163 to Bhate and Vitale. Similarly, U.S. Pat. No. 4,937,481 to Vitale discloses a linear electromagnetic machine including a plunger having a plurality of magnetic segments disposed axially and of alternating polarity (i.e., the term "magnetic strength" as used herein shall refer to the `absolute` magnitude and not the polarity of the magnet).
Magnetic self-centering of the moving magnet plunger in a linear motor is highly desirable so that start-up will occur from the mid-stroke position and will occur without hitting the end-of-stroke stops. In a typical three-magnet design, such as disclosed in U.S. Pat. No. 4,937,481, the two wing magnets extend beyond the pole pieces of the outer stator when the plunger is at mid-stroke. As a result, this configuration is self-centering because of flux leakage from the portions of the wing magnets that extend beyond the outer stator.
Typically, all three magnets are made from the same high strength permanent magnet material which is usually quite expensive. However, the center magnet is the one that is most effective in providing the flux changes needed in the motor to generate power when the coil is energized with AC current. Eliminating the wing magnets, or reducing their strength significantly, will not drastically reduce the motor's power capability or efficiency, but wing magnets of some strength are needed to provide the plunger with self-centering capability. If weaker wing magnets are substituted, then the motor could be brought back to full power capability by using a somewhat stronger magnet in the center position. If substantially different performance or behavior is desired, then the magnet in the center position could be of lower strength relative to the wing magnets or could be replaced by either high permeability material or by an air gap (or material having magnetic properties similar to an air gap).
It is therefore an object of this invention to reduce the magnetic strength, and hence the cost, of the wing magnets or center magnets of a linear motor or alternator.